Mechanical Alloying: An Advantageous Method for the Development of Mg₂Si_0.8Sn_0.2 and Mg₂Si Thermoelectrics Using Commercial and Recyclable Silicon

A comparative study of Bi-doped Si-rich silicide phases, Mg₂Si_0.8Sn_0.2 and Mg₂Si, is reported, investigating in parallel two different synthetic routes: the solid-state reaction (SSR) and mechanical alloying (MA). Both synthetic routes produce the desired silicide phases. However, powder XRD Rietveld refinements reveal appreciable Mg and Sn losses for the SSR-developed Mg₂Si_0.8Sn₂, while EDS measurements also confirm Sn losses together with a decrease in the Bi content. This has a strong impact in electrical transport properties, indicating a severe electron doping deficiency. In contrast, the EDS results for MA-based phases are in a good agreement with the nominal values, indicating an effective Bi doping. Moreover, considering the Rietveld refinement results and SEM analysis, notable changes in the content of Mg interstitial atoms at the 4b crystallographic site seem to be correlated with the microstructure features of the two MA compounds. Electrical conductivity and Seebeck coefficient measurements confirm the aforementioned results. In addition, a small reduction in lattice thermal conductivity is observed for the two MA systems due to the nanostructuring effect. At 773 K, ZT values of 0.85 and 0.6 are exhibited for Mg₂Si_0.8Sn_0.2 and Mg₂Si, respectively. MA is proven to be an advantageous route for the development of Si-rich phases since it provides a better control of doping and higher precision of produced stoichiometric compositions, while in parallel it is a straightforward and scalable method. The replacement of commercial Si by two types of recycled Si-kerf is also attempted here. The kerf-based materials exhibit small reductions in ZT, giving prominence to the efforts to utilize more effectively recyclable Si.